Allomorph (idea)

• The "er" in "rider" and the "or" in "actor" (allomorphs of {-er}, a bound derivational suffix meaning "one who does <something>")
• The "s" in "dogs" and the "es" in "coaches" (allomorphs of {-s}, the bound inflectional suffix indicating plurality)

A special case of this is the zero allomorph, used to denote changes in form which are invisible. An example of this is "sheep" (singular) and "sheep" (plural); morphologically, the latter would be written {sheep} + {0}, to illustrate the presence of the English inflectional suffix {-s}, even though it is not visible.